Organic light emitting devices (OLEDs) are useful for display applications and in particular, mobile hand held display applications. To achieve efficient electroluminescence OLEDs have typically been manufactured to include separate layers of a hole transport material (HTM) and an emitting electron transport material (ETM). During operation, an applied electric field causes positive charges (holes) and negative charges (electrons) to be respectively injected from the anode and the cathode of the OLED to recombine and thus produce light emission. In other known OLED displays, the hole transport and electron transport layers are doped with organic dyes in order to enhance the efficiency or to improve the stability of the OLED. OLEDs have also been developed in which hole transport material and emitting electron transport material are mixed together in one single layer. However, such single mixed layer organic light emiting devices are generally less efficient an unstable compared to multilayer organic light emitting devices. There have also been attempts to obtain electroluminescence from organic light emitting devices by introducing hole transport material and emitting electron transport material as dopants in an inert host material. However, such known devices have been found to be generally less efficient than conventional devices including separate layers of hole transport material and emitting electron transport material.
To date, organic light emitting devices as described above have relatively short operational lifetimes before their luminance drops to some percentage of its initial value. Methods of providing interface layers and doping have been developed to increase the operational lifetime of organic light emitting devices to several tens of thousands of hours for room temperature operation, however, the effectiveness of the known organic light emitting devices deteriorates dramatically for high temperature device operation, as the existing methods used to extend the device lifetimes lose their effectiveness at higher temperatures.
A simple organic electroluminescent (EL) device may comprise of a layer of an organic luminescent material conductively sandwiched between an anode, typically comprised of a transparent conductor, such as indium tin oxide, and a cathode, typically a low work function metal such as magnesium, calcium, aluminum, or the alloys thereof with other metals. The EL device functions on the principle that under an electric field, positive charges (holes) and negative charges (electrons) are respectively injected from the anode and cathode into the luminescent layer and undergo recombination to form excitonic states which subsequently emit light. A number of prior art organic electroluminescent devices have been prepared from a laminate of an organic luminescent material and electrodes of opposite polarity, which devices include a single crystal material, such as a single crystal anthracene, as the luminescent substance as described, for example, in U.S. Pat. No. 3,530,325. However, these devices usually require excessive excitation voltages on the order of 100 volts or greater.
An organic EL device can also be formed as a dual layer structure comprising one organic layer adjacent to the anode supporting hole transport, and another organic layer adjacent to the cathode supporting electron transport and acting as the organic luminescent zone of the device. Another alternate device configuration is comprised of three separate layers, a hole transport layer, a luminescent layer, and an electron transport layer, which layers are laminated in sequence and are sandwiched between an anode and a cathode. Optionally, a fluorescent dopant material can be added to the emission zone or layer whereby the recombination of charges results in the excitation of the fluorescent.
U.S. Pat. No. 4,539,507 discloses an EL device formed of a conductive glass transparent anode, a hole transporting layer of 1,1-bis(4-p tolylaminophenyl)cyclohexane, an electron transporting layer of 4,4′-bis(5,7-di-tert-pentyl-2-benzoxzolyl)stilben, and an indium cathode.
U.S. Pat. No. 4,720,432 discloses an organic EL device comprising a dual-layer hole injecting and transporting zone, one layer being comprised of porphyrinic compounds supporting hole injection and the other layer being comprised of aromatic tertiary amine compounds supporting hole transport.
U.S. Pat. No. 4,769,292 discloses an EL device employing a luminescent zone comprised of an organic host material capable of sustaining hole-electron recombination and a fluorescent dye material capable of emitting light in response to energy released by hole-electron recombination. A preferred disclosed host material is an aluminum complex of 8-hydroxyquinoline, namely tris(8-hydroxyquinolinate)aluminum.
U.S. Pat. No. 6,392,339 (the entirety of which is incorporated herein by reference) describes an OLED manufactured to provide operational stability at high temperature device operation conditions. The OLED comprises a mixed region or layer of a hole transport material and electron transport material having first and second surfaces, an electron transport material provided on one of the surfaces of the mixed region and an optional hole transport material on one of the other surfaces of the mixed region, where an anode is further provided in contact with the mixed region or the hole transport material and a cathode is provided in contact with the electron transport material or with a first surface of the mixed region.
Typically, the organic EL devices with multi-layered configurations comprise an electron transport layer in contact with a cathode. This electron transport layer is intended to assist injection of electrons from the cathode into the light-emitting layer. A variety of organic electron transport materials have been employed for this purpose. A class of such electron transport materials is comprised of the metal complexes of 8-hydroxyquinoline, as disclosed in U.S. Pat. Nos. 4,720,432. A another class of electron transport materials for EL devices is comprised of 1,3,5-oxidiazole compounds, such as those disclosed in Japanese Journal of Applied Physics, Part 2, vol. 34, L824 (1995). Also, certain 1,3,5-triazine containing materials have been reported as being a hole blocking layer in organic EL devices, see Fink et al. in Macromolecular Symposia, vol. 125, 151(1997).
While recent progress in organic EL research has elevated the potential of organic EL devices for widespread applications, the performance levels of current available devices may still be below expectations. Further, for visual display applications, organic luminescent materials should provide a satisfactory color in the visible spectrum, normally with emission maxima at about 460, 550 and 630 nanometers for blue, green and red. The aforementioned metal complexes of 8-hydroxyquinoline, such as tris(8-hydroxyquinolinate)aluminum, generally fluoresce in green or longer wavelength region. These electron transport materials may be suitable for use in EL devices with light emission in green or longer wavelength region, however, for blue-emitting EL devices they are of limited use. Although prior art electron transport materials may fluoresce in the blue region, the performance characteristics of the resulting EL devices still possess many disadvantages such as poor operation stability.
Thus, there continues to be a need for electron transport materials for organic EL devices, which are suitable for the design of EL devices with satisfactory emission in the visible spectrum of from blue to longer wavelength region. There is also a need for electron transport materials, which can improve EL device operation stability and durability, and a need for electron transport materials, which can enhance the EL charge transporting characteristics, thus lowering device driving voltages. Further, there is a need for electron transport materials, which are vacuum evaporable and form thin films with excellent thermal stability.